A tandem-type full-color image forming apparatus provided with a plurality of image bearing members respectively bearing developer images of different hues includes a transfer device to transfer an image onto a recording medium through an endless belt (for example, refer to the Patent Literature 1). The endless belt is installed in the image forming apparatus in such a manner that a predetermined region of its outer circumferential surface faces the plurality of image bearing members. The transfer device includes a plurality of transfer members opposed to the plurality of image bearing members across the endless belt.
In such a transfer device, when a monochromatic image forming is performed, the transfer member for monochrome is caused to approach the image bearing member for monochrome while the transfer members for color are caused to be separate from the respectively corresponding image bearing members for color. This causes the endless belt to come into contact with the image bearing member for monochrome, while causing it not to come into contact with the image bearing members for color. Then, when a full-color image forming is performed, the transfer member for monochrome and the transfer members for color are respectively caused to approach the respective plurality of corresponding image bearing members. This causes the endless belt to come into contact with all the image bearing members.
Application of a transfer bias to the transfer members corresponding to the image bearing members that have come into contact with the endless belt causes the developer images to be transferred from the respective image bearing members onto the endless belt.